Conventionally, an aerobic biological treatment method, which is referred to as an activated sludge method, has been used to treat organic waste water. This method is carried out as shown in FIG. 8, wherein organic waste water, such as sewage, introduced from organic waste water reservoir 1 into biological treatment tank 3 is decomposed into an inorganic matter, such as carbon dioxide or water, under aerobic conditions in biological treatment tank 3 by biological oxidation, which is oxidative decomposition performed by a microorganism. The waste water which has been treated in the biological treatment tank 3 is solid-liquid separated into treated water C and sludge D in precipitation tank 5. In general, a portion of sludge D is returned as a microorganism source to biological treatment tank 3, and the residual sludge is treated as an excess sludge E.
In this case, however, precipitated and concentrated sludge containing an organic solid, which has been solid-liquid separated in the precipitation tank 5, is treated through the steps of concentration, digestion, dehydration, composting, and incineration. Such a treatment is not preferred because it requires a large quantity of labor, time, and expense.
For this reason, methods of decreasing a sludge as much as possible, such as an extended aeration method for increasing the residence time of the sludge, and a fixed bed oxidation method utilizing a biomass attached to a catalytic material surface of a media to hold a large quantity of microorganisms in a reaction tank (see "Sewage Service Project--Design Guide and Explanation" issued by Japan Sewage Works Agency, edited by Sewage Works Division, Municipal Department of the Ministry of Construction, Vol. 2, 1994), have been proposed and commercialized. However, these methods require a large site area for increasing the residence time. In the extended aeration method, the sludge is dispersed when a load is reduced, thereby interfering with solid-liquid separation. In the fixed bed oxidation method, fouling of the sludge results when the load is increased. Therefore, these methods are not desirable. In order to solve these problems, a method has been proposed which temporarily stores an excess sludge, and decreases the sludge by an anaerobic digestion method to reduce the quantity of the sludge and to lessen the waste treatment load. In this method, the treatment time is long, for example 20 to 30 days, and the decrease of organic sludge is not sufficient, for example 30% to 50%.
Japanese Provisional Patent Publication No. 6-206088 discloses a method for performing an aerobic biological treatment on organic waste water, and then oxidizing a solid-liquid separated sludge by an ozone oxidation column, to reduce an excess sludge. However, this method requires operators well-skilled in handling the ozone oxidation column, and has the problem of treating residual ozone. In addition, the decomposition rate of the excess sludge is not sufficient in the ozone oxidation column.
Japanese Provisional Patent Publication No. 9-10791 discloses an activated sludge treatment method capable of considerably reducing the quantity of a generated excess sludge. The method comprises steps of subjecting organic waste water to an aerobic biological treatment in an aeration tank, solid-liquid separating the waste water treated in the aeration tank into treated water and a sludge in a precipitation device, returning a part of the sludge separated in the precipitation device to the aeration tank through a recycle route, solubilizing the excess sludge separated in the precipitation device at a high temperature in a solubilizing treatment device, and returning the treated sludge solubilized in the solubilizing treatment device to the aeration tank through a return route. According to the method described in the publication, however, it is necessary to increase the size of the solubilizing treatment device according to the maximum quantity of the excess sludge which flows into solubilizing treatment equipment. Furthermore, a large quantity of treated sludge solubilized in the solubilizing treatment device is returned to the aeration tank. Therefore, the substantial residence time is shortened in the aeration tank. Consequently, the quality of treated water at times is reduced.
Japanese Provisional Patent Publication No. 9-276887, as shown in FIG. 9, discloses an organic waste water treatment apparatus comprising an activated sludge treatment tank 21, a solid-liquid separation device 22 for solid-liquid separating a sludge obtained after an activated sludge treatment, a sludge returning means 23 for returning a part of the separated sludge to activated sludge treatment tank 21, a sludge concentration device 24 for concentrating the residual sludge, a heating device 25 for heating the concentrated sludge to a temperature of 40 to 100.degree. C., and a sludge returning means 26 for returning the heated sludge to activated sludge treatment tank 21. According to the treatment apparatus, the excess sludge separated in solid-liquid separation device 22 is concentrated in sludge concentration device 24, and then is sent to heating device 25. There is therefore the advantage that the quantity of the sludge sent to heating device 25 can be decreased. In the treatment apparatus, however, the sludge is solubilized in only a heating treatment. For this reason, the solubilization ratio of the sludge is low, for example 20% to 25%. It should take a long time to solubilize a large quantity of sludge by using a solubilizing tank of a large capacity. Consequently, the scale of equipment is enlarged. Thus, the treatment apparatus has a disadvantage that the cost of heating, the maintenance cost, and the like are increased.
In order to reduce the equipment cost, furthermore, it is preferable to reduce the number of required devices and the space occupied by the devices.